6 



A New Method of Treating 



[APRIL, 



The meteorological characteristics of the current season and 

 the past four seasons are summarised by enumerating the number 

 of weeks of each kind which they contain. 



The adjectives are to be understood as describing the diver- 

 gence of the weekly values from the average " for the time of 

 year." At first sight it might appear reasonable to regard all 

 values as " moderate " which lie within a fixed limit of the 

 average. Thus we might agree to call a week's rainfall moderate 

 if it were within, say, o - 1 inch of the average for the time of year 

 and to use the terms heavy or light if it differed from it by more 

 than this amount. The following considerations however 

 show that such a system would not give satisfactory results 

 when the number of weeks of each kind in a lengthy period 

 come to be enumerated. 



If the rainfall statistics for past years be examined from 

 the point of view of the frequency of occurrence of falls of given 

 magnitude, the weekly totals are found to group themselves 

 very unevenly with respect to their average. A single very 

 heavy fall compensates for a large number of small falls with 

 the result that the average is greater than the " median " or 

 middlemost value when the falls are arranged in order of magni- 

 tude. The middlemost value of a group of numbers is to be 

 understood as that which is so related that there are as many 

 above it in magnitude as there are below it. We may say that 

 in rainfall statistics, the middlemost value falls so that the 

 chance that a value chosen at random is less than the average 

 is considerably greater than the chance that it is above it. It 

 is not correct to regard the average as " what we may expect," 

 as is so often done implicitly, if not explicitly. 



An example will make this point clear. For illustration we 

 select the rainfall in District 3, " England East," during the 13 

 weeks which make up Spring (10th to 22nd weeks of the year). 

 The statistics for 25 years give us 25 x 13 = 325 values of 

 weekly rainfall within this period. If the frequency distribution 

 of these be determined for gradations of o-i in., they group 

 themselves as follows : — 



